Source : MacFixitSome people have experienced a noticeable slowdown when copying large files over an ethernet network, since their upgrade to Leopard. MacFixit has a solution for this problem: enter the following command in Terminal:
sudo sysctl -w net.inet.tcp.delayed_ack=0
(The system will prompt for an administrator password)
Pay attention, this modification will be reset after reboot. If you want it to stay forever, download the following script and let it auto-launch when your computer boots up: http://systemsboy.blogspot.com/
[translation by xavier]
